1. Field of the Invention
This invention relates to a start-stop synchronous data transmission system, and in particular, to such a system wherein a main equipment receives a time division multiplex signal from an external equipment through a common transmission line or a highway, and sends out the digital data signals of a plurality of channels in the multiplex signal to a single transmission line as the start-stop synchronous data signals to transmit the data signals to desired ones of a plurality of sub-equipment, respectively.
2. Description of the Prior Art
It is well known in the prior art to transmit a plurality of digital data signals from a main equipment to a plurality of sub-equipments, respectively, through a common transmission line according to the start-stop synchronous system. It is natural that an address data is added to each digital data signal as well as the start and stop bits.
Those digital data signals are produced within the main equipment in some systems, and they are supplied to the main equipment by a time division multiplex signal from an external equipment in another systems such as, for example, a digital private branch exchange (DPBX) system.
In a known DPBX system, the DPBX apparatus is provided with a transmitter which is connected to a plurality of subscribers through a common digital transmission line. Digital data signals of a plurality of channels are supplied to the transmitter as the time division multiplex signal through a highway in the DPBX. In the transmitter, the address data, the start bit and the stop bit are added to each digital data signal of each channel in each frame of the time division multiplex signal as received, and the resultant digital signal is sent out to the common digital transmission line as a time division multiplex signal. Each subscriber discriminates a channel signal addressed thereto according to the address data, and derives the digital data signal in the channel signal.
In the known DPBX system, the time division multiplex signal on the common digital transmission line is quite high in the redundancy because many redundant bits such as the address data and the start and the stop bits are added to every digital data in every channel.